Apparatus and Method for Elevated Temperature Electrospinning

ABSTRACT

Elevated temperature electrospinning apparatus comprises a pump upstream of or containing a resistance heater, means to shield applied electrostatic field from the resistance heater, and a temperature modulator for modulating temperature in the spinning region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/583,358 filed Jun. 29, 2004, the whole of which isincorporated herein by reference.

TECHNICAL FIELD

This invention is directed to relationship of elevated temperatureelectrospinning apparatus components, including isolation of the chambersupplying heat for melting and temperature control in the spinningregion.

BACKGROUND OF THE INVENTION

Fibers with diameters less than a micron can be formed usingelectrospinning processes where a droplet of polymer solution or melt iselongated by a strong electrical field. The resulting fibers arecollected as non-woven mats with extremely large surface to volumeratio; which are useful for various applications including filtration.Most previous studies on electrospinning have focused on fibers frompolymer solutions, i.e., are directed to solution electrospinning.Current solution electrospinning apparatus and processes have thedisadvantages of requiring a dissolving step, of requiring solventrecovery and disposal or complete recycling if the process is to beenvironmentally friendly, of having low production rates because of thedissolving and solvent recovery/recycling steps detracting fromobtaining high throughput, of not being adaptable to polymers such aspolyethylene, polypropylene, polyethylene terephthalate and polybutyleneterephthalate, which are not dissolvable in acceptable solvents at roomtemperature, of requiring regulation of a plurality of parameters toadjust molecular properties and solidification and of requiringapparatus not readily provided by adaption of conventional existingfacilities for fiber/non-woven production for most polymers since theseare based on melt treatment. Melt electrospinning apparatus and processwhich would avoid these disadvantages and provide useful production offibers/non-wovens have not heretofore been developed. Moreover, noattempts have been made to provide solution electrospinning apparatusand processes which are suitable for operation on polymers which are notdissolvable in acceptable solvents at room temperature.

SUMMARY OF THE INVENTION

It has been discovered herein that apparatus and process avoiding thedisadvantages of conventional solution electrospinning apparatus andprocess and providing useful melt electrospinning production of polymerand nanocomposite fibers/non-wovens and useful solution electrospinningapparatus and processes for operation on polymers which are notdissolvable in acceptable solvents at room temperature, can be providedby relying on unique heating apparatus/process.

In one embodiment herein, the invention is directed to apparatus forelevated temperature production of non-woven fabric from thermoplasticpolymer or thermoplastic polymer nanoclay nanocomposite, neat or insolution and requiring elevated temperature for dissolving in anacceptable solvent, said apparatus comprising a resistance heater formelting the polymer or nanocomposite or maintaining the polymer ornanocomposite in solution in acceptable solvent; a pump upstream of orcontaining the resistance heater for dispensing said melted polymer ornanocomposite or solution; a droplet forming passageway for receivingsaid polymer or nanocomposite melt or solution and having one or moreoutlet orifices for providing one or more droplets of melted polymer ornanocomposite or solution at the one or more outlet orifices; a guidingchamber having an inlet side in fluid communication with the outletorifice(s); a collection surface at the rear side of the guiding chamberfor receiving elongated fibers of polymer or nanocomposite andcollecting them as a non-woven fabric; and a high voltage source inelectrical communication with the droplet forming passageway to providean electric charge in the droplet(s) emitting therefrom to overcome thesurface tension of a droplet to produce a jet of melted polymer ornanocomposite or solution in the guiding chamber giving rise to unstableflow through the guiding chamber to the collection surface manifested bya series of electrically induced bending instabilities and flashing offof any solvent during passage of polymer or nanocomposite to thecollection surface and production of elongated fibers of polymer ornanocomposite and deposit of these on the collection surface so as toform the non-woven fabric. In the apparatus preferably, the electricalcommunication of the high voltage source is shielded from the resistanceheater to prevent induced voltage in the resistance heater and atemperature modulator is provided for the guiding chamber to adjustcooling of the fiber being formed to provide against prematuresolidification and to provide against induction of relaxation ofmolecular orientation, and to potentiate flashing off of any solvent,without affecting the bending instabilities causing fiber elongation.

The term “elevated temperature” as used herein, refers to meltelectrospinning production, or solution electrospinning production in anacceptable solvent at a temperature ranging from 50° C. to 250° C.; forsolution electrospinning of polyolefins a temperature ranging from 120°C. to 180° C. is preferred.

The term “acceptable solvent” as used herein, means a solvent satisfyingthe following requirements: (i) the solubility is higher at elevatedtemperature than at room temperature; (ii) the flashpoint is below thespinning temperature; (iii) the solvent is sufficiently volatile so asto evaporate during the spinning process; and (iv) the solvent's odorthreshold level is higher than 0.1 ppm.

Said apparatus which is for batch operation can comprise the followingelements:

(a) a syringe having an inlet for introduction into the syringe of solidmeltable thermoplastic polymer or solid meltable thermoplastic polymernanoclay nanocomposite or solution of thermoplastic polymer orthermoplastic polymer nanoclay nanocomposite requiring elevatedtemperature for dissolving in an acceptable solvent, and an outlet fordispensing of melted thermoplastic polymer or nanocomposite or elevatedtemperature solution,

(b) a heating chamber in heat exchange communication with the syringe tosupply heat to the syringe to melt polymer or nanocomposite or maintainpolymer or nanocomposite in solution within the syringe,

(c) droplet forming passageway having an inlet in fluid communicationwith the outlet of the syringe and one or more outlet orifices forproviding one or more droplets of polymer or nanocomposite melt orelevated temperature solution at the one or more outlet orifices;

(d) a pump upstream of the inlet of the syringe for causing the syringeto dispense melted polymer or nanocomposite or elevated temperaturesolution to be electrically charged,

(e) a guiding chamber having inlet side in fluid communication with theorifice outlet(s);

(f) a collection surface at a rear end of the guiding chamber; and

(g) a high voltage source in electrical communication with the dropletforming passageway to provide an electric charge in the formeddroplet(s) emitting therefrom to overcome the surface tension of adroplet to produce a jet of melted polymer or nanocomposite or elevatedtemperature solution in the guiding chamber giving rise to unstable flowthrough the guiding chamber to the collection surface manifested by aseries of electrically induced bending instabilities. i.e., whippingmotions, and flashing off of any solvent, during passage to thecollection surface, and production of elongated fibers of the polymer ornanocomposite which are deposited on the collection surface where theyare collected as a non-woven fabric.

In a preferred case, said apparatus for batch operation also comprisesat least one of the following elements of (h), (i) and (j) or any two ofthe elements, and preferably all of the following elements (h), (i) and(j):

(h) a temperature modulator for the guiding chamber to adjust cooling ofthe fiber being formed to provide against premature solidification andto provide against relaxation of induction of molecular orientation andto potentiate flashing off of any solvent, without affecting the bendinginstabilities causing fiber elongation,

(i) a controller for controlling temperature in the heating chamber, aheating coil in the heating chamber, and shielding for the heating coilinside the heating chamber to prevent induced voltage in the heatingcoil from the electric charge supplied by the high voltage source sothat induced voltage will not affect or damage the controller, and

(j) the heating chamber being constructed of material comprising asubstance that provides both thermal and electrical insulation.

Very preferably, the apparatus for batch operation also comprises amodulator for the temperature of the collection surface to provideannealing of fibers deposited and collected on the collection surface toprovide fibers on the collection surface with properties that do notchange with time and increased molecular orientation such as increasedcrystallinity.

Said apparatus which is for continuous melt electrospinning operationand for production of non-woven fabric from thermoplastic polymer orthermoplastic polymer nanoclay nanocomposite, can comprise a hopper forcontaining and feeding chunks of thermoplastic polymer or thermoplasticpolymer nanoclay nanocomposite; an extruder for receiving the chunks ofpolymer or nanocomposite and conveying, melting and pumping the polymeror nanocomposite to produce a flow of polymer or nanocomposite melttherefrom; a melt pump for receiving polymer or nanocomposite melt fromthe extruder and for maintaining the melted condition of the polymer ornanocomposite melt by means of electric resistance heating and providinga melt output; a header (manifold) for receiving the melt output anddistributing it to multiple nozzles for forming droplets of polymer ornanocomposite melt; a guiding chamber for receiving the output of thenozzles; a collection surface at the rear of the guiding chamber; a highvoltage source in electrical communication with the nozzles to providean electric charge in the droplets emitting therefrom to overcome thesurface tension of a droplet to produce a jet of polymer ornanocomposite melt giving rise to unstable flow through the guidingchamber to the collection surface manifested by a series of electricallyinduced bending instabilities during passage to the collection surfaceand production of elongated fibers of the polymer or nanocomposite whichare deposited on the collection surface where they are collected as anon-woven fabric; a shield for the header and nozzles to prevent inducedvoltage in the melt pump from the electric charge supplied by the highvoltage source; and an infrared heater for the guiding chamber to adjustcooling of the fiber formed therein to provide against prematuresolidification and to provide against induction of relaxation ofmolecular orientation, without affecting the bending instabilitiescausing fiber elongation.

In a second embodiment herein, the invention is directed at a method formelt electrospinning production of nonwoven fiber from meltablethermoplastic polymer or meltable thermoplastic polymer nanoclaynanocomposite, said method comprising the steps of:

(a) melting thermoplastic polymer or nanocomposite in a melting zone,

(b) moving the thermoplastic polymer or nanocomposite through themelting zone by force supplier upstream of or in the melting zone,

(c) forming droplets of the melted polymer or nanocomposite,

(d) providing an electric charge on the droplets to overcome the surfacetension of a droplet to produce a jet of melted polymer or nanocompositeand provide unstable flow involving a plurality of electrically inducedbending instabilities/whipping motions and elongation of and productionof polymer or nanocomposite fibers,

(e) collecting of elongated fibers to form a non-woven fiber.

A preferred method of said second embodiment also comprises at least oneof the following elements (f) and (g) and very preferably both of thefollowing elements (f), and (g):

(f) providing a temperature for the polymer or nanocomposite beingsubjected to electrically induced bending instabilities/whipping motionsand elongation to provide against premature solidification and toprovide against induction of relaxation of molecular orientation withoutaffecting the electrically induced bending instabilities,

(g) shielding to prevent induction voltage of in the melting zone.

Very preferably the method of said second embodiment also comprises theadditional step of annealing the collected fibers to impart stabilityand molecular orientation thereto.

In a third embodiment herein, the invention is directed at a method forhigh temperature solution electro spinning of non-woven fabric fromthermoplastic polymer or thermoplastic polymer nanoclay nanocompositethat is not dissolvable at room temperature in an acceptable solvent,said method comprising the steps of

(a) homogenizing the polymer or nanocomposite in solvent in an elevatedtemperature zone to form a solution of the polymer or nanocomposite inthe solvent;

(b) maintaining the solution at a temperature sufficient for maintainingdissolution in a second elevated temperature zone;

(c) moving the solution through the second elevated temperature zone bya force supplier upstream of or at the second elevated temperature zone;

(d) forming droplets of the solution moved through the second elevatedtemperature zone;

(e) providing an electric charge on the droplets to overcome the surfacetension of a droplet to produce a jet of polymer or nanocompositesolution and provide unstable flow involving a plurality of electricallyinduced bending instabilities/whipping motions and flashing off ofsolvent and elongation of and production of polymer or nanocompositefibers;

(f) collecting the fibers to form a non-woven fabric.

Preferably, the method of the third embodiment comprises at least one ofthe following steps (g) and (h) and ver preferably both of the followingsteps (g) and (h):

(g) providing a temperature for the polymer or nanocomposite andsolution thereof being subjected to electrically induced bendinginstabilities/whipping motions and fiber elongation to provide againstpremature solidification and to provide against induction of relaxationof molecular orientation and potentiate flashing off of solvent, withoutaffecting the electrically induced bending instabilities,

(h) shielding to prevent induction of voltage in the second elevatedtemperature zone.

Very preferably, the method of the third embodiment comprises theadditional step of annealing the collected fibers to impart stabilityand molecular orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic description of elevated temperatureelectrospinning apparatus of the invention herein for batch operation.

FIG. 2 is a schematic depiction of melt electrospinning apparatus of theinvention herein for continuous operation.

DETAILED DESCRIPTION

To aid in the understanding of melt electrospinning, the followingdiscussion is provided.

An electric charge is generated on a formed suspended drop of meltedpolymer or nanocomposite. This charge overcomes the surface tension ofthe suspended drop to produce an electrically charged jet of meltedpolymer or nanocomposite which undergoes a series of electricallyinduced bending instabilities whereby repulsion of adjacent chargedsegments generates vigorous whipping motion during passage to acollection surface resulting in significant elongation and stretching ofthe produced fiber. The stretched fibers are accumulated on the surfaceof a collection plate resulting in nonwoven fabric including mesh ofnanometer to micron diameter fibers. Varying of the electric fieldstrength/electric charge, drop forming nozzle orifice temperature,nozzle diameter, flow rate, distance from nozzle to collection plate andtemperature during elongation, controls the fiber diameter.

In the elevated temperature solution electrospinning herein thedifference from the above paragraph is that the suspended drop is ofelevated temperature polymer or nanocomposite solution. The whippingaction described above occurs in electrically charged jet of solutionjust as in electrically charged jet of melt because of variation ofsurface charges and electric field which occur in a solution as well asin a melt. A difference from melt electrospinning is that solventflashes off during fiber formation and elongation and is removed fromthe system. Variation of electric field strength/electric charge, nozzleorifice temperature, nozzle diameter, flow rate, distance from nozzle tocollecting plate and temperature during elongation, controls the fiberdiameter.

We turn now to the polymer and nanocomposite which can be processed inapparatus of the invention by means of melt electrospinning operation.The polymer can be any meltable thermoplastic polymer includingamorphous and crystallizing polymers, e.g., amorphous polymers such asrubber, polycarbonate, polystyrene and poly(methyl methacrylate); slowcrystallizing polymers such as poly(lactic acid) denoted PLA; mediumcrystallizing polymers such as polyethylene terephthalate; fastcrystallizing polymers such as polybutylene terephthalate, nylon 6,polypropylene and polyethylene; and very fast crystallizing polymerssuch as nylon 6,6.

As used herein, the term “nanocomposite” means composition of nanoclayin a polymer matrix containing by weight, for example, up to 20%, e.g.,1 to 10%, nanoclay.

The term “nanoclay” means clay having nanometer thickness silicateplatelets that can be modified to make clay compatible with organicmonomers and polymers, i.e., by cation exchanging nanoclay, e.g., asobtained in the sodium form, with organic cation. The nanoclay can be,for example, montmorillonite (a natural clay) or fluorohectorate orlaponite synthetic clays. Other useful nanoclays include, for example,bentonites, beidellites, hectorites, saponites, nontronites, sauconites,vermiculites, ledikites, magadiites, kenyaites and stevensites.Processes for making polymer/clay nanocomposites are known and have beenpatented and are under commercial development.

We turn now to polymer solutions which can be processed in apparatus ofthe invention by means of solution electrospinning operation. Thepolymers for the solutions as indicated above, are polymers which arenot dissolvable in acceptable solvents at room temperature. Theseincluded polyolefins, e.g., polyethylene, polypropylene andpolysobutylene, which are not dissolvable in any solvents at roomtemperature, but are dissolvable at elevated temperatures as describedabove. Suitable solvents for use in providing solutions of polyolefinsat 100 to 180° C. for solution electrospinning herein include, forexample, decalin, paraffin oil, ortho dichlorobenzene and xylene.Polymers which are dissolvable at room temperature, but for which noacceptable solvents are available for dissolving at room temperature,are some polyesters, e.g., polyethylene terephathalate (PET). While PETis readily dissolved at room temperature in phenol, residual phenol ispresent and is a problem even at a few parts per million, as it ispoisonous and caustic and is readily absorbed through skin, and from thestomach and lungs. Acceptable solvents for PET at elevated temperaturesof 50 to 200° C., include for example, toluene, benzene, chlorobenzeneand xylene/chlorohexanone.

As it is clear from the above, polyolefins and polyethyleneterephthalate can be used as polymer for either melt electrospinningoperation or for elevated temperature solution electrospinningoperation. In these cases, elevated temperature solution electrospinningmay be preferred, because nanoscale diameter fibers can more easily beobtained with high temperature solution electrospinning that with meltelectrospinning.

We turn now to apparatus of the invention herein involving batchoperation.

With continuing reference to FIG. 1 of the drawings, there is depicted aheating chamber 10 containing an electrical resistance heating element(not shown), e.g., a heating coil. The heating chamber 10 is in heatexchange contact with a syringe 11, e.g., of circular cross-section ofone-half to one inch diameter, which extends through chamber 10 with itslongitudinal axis oriented horizontally. The syringe 11 is to housepolymer or nanocomposite to be melted or elevated temperature solutionof polymer or nanocomposite to be maintained at elevated temperature,and melted polymer or nanocomposite or elevated temperature solution ofpolymer or nanocomposite to be dispensed. The syringe 11 contains aplunger 13 at its inlet end for removal for introduction of solidpolymer or nanocomposite or elevated temperature solution of polymer ornanocomposite and followed by reinsertion and movement forward to movepolymer or nanocomposite or said elevated temperature solution firstinto heat exchange contact for melting of said polymer nanocomposite ormaintaining the elevated temperature of polymer or nanocompositesolution and thereafter further forward for dispensing of melt orelevated temperature solution through a dispensing end 14. Thetemperature in the syringe, denoted T₁, is controlled by a temperaturecontroller 12 to provide temperature in the heating element to controlthe viscosity of molten polymer or nanocomposite or elevated temperaturesolution being dispensed to one that will provide droplets of polymer ornanocomposite or polymer or nanocomposite as described later (e.g., atemperature of 200° C. for PLA). As indicated at 15 a thermocouple incommunication with controller 12 is placed in chamber 10 to provide afeedback mechanism. The heating chamber is shown to contain a window 16to allow visual access to the inside of the chamber 10 and of thesyringe 11 to determine the presence of sparks and leakage and theextent of melting of polymer or nanocomposite in syringe 11. The wallsof heating chamber 10 are preferably constructed of a material thatprovides both thermal insolation (to provide heating efficiency) andelectrical insulation (to prevent leakage currents from applied highvoltage, as described later, from entering the heating chamber, e.g., amaterial based on CaSiO₃, or a ceramic composite; glass also works.Movement of the syringe plunger 13 forward, e.g., by a mini-pump,connected to plunger 13, provides horizontal displacement of plunger 13to continuously dispense droplets of polymer or nanocomposite orelevated temperature solution of polymer or nanocomposite as describedlater. A droplet forming passageway 20 having an inlet in fluidcommunication with the dispensing end 14 of syringe 11 and one or moreoutlets orifices (capillary tips) for providing one or more droplets ofliquid polymer or nanocomposite at the one or more outlet orifices, isprovided by a needle (e.g., a 24 gauge needle) or spinneret. A highvoltage supplier 22 is present to supply high voltage (a typical voltageis 10 kV to 30 kV where the distance between the syringe tip/orificeoutlet(s) and collector as described later is 2 to 10 inches) via aconductive element 23 to the syringe tip/orifice outlets to provide anelectrostatic field strength, e.g., of 1 to 10 kV/cm, where cm refers tothe distance between the droplet forming orifice of passageway 20 andthe collector 28, to drive the flow of polymer or nanocomposite orelevated temperature solution and whipping action as described later.The resistance heating coil in heating chamber 10 is preferablyprotected from induction of voltage therein from said electrostaticfield since induced voltage can affect the accuracy of or damage thecontroller 12. In addition to the electrically insulating material ofconstruction of heating chamber 10, this is preferably provided bysurrounding any heating coil in chamber 10 with an electrostaticshielding element (not shown), very preferably, a Faraday cage, alsocalled a Faraday screen or Faraday shield, which is an enclosuresurrounding the heating coil and made of screening, e.g., metal mesh ofmesh size #5, which wraps around the heating coil without touching it,electrically attached to earth ground with a conductive wire. TheFaraday cage eliminates any induced electrostatic voltage on the coilinside the cage. In the unit where runs were carried out herein, thecoil and cage are positioned in parallel with the vertical walls ofheating chamber 10. The temperature in the orifice forming passageway,denoted T₂, is preferably regulated and fine tuned, by use of acylindrical heater as indicated at 41 electrically shielded in a ceramiccylinder, or by use of circulating hot air (elements(s) for providingthis are not shown) to control the viscosity of the fluid exiting thepassageway 20. The temperature T₂ is controlled by a controller 40 withfeedback via 42 in response to results at the needle/spinneret 20. Withincreasing T₂, the viscosity decreases. Too high a viscosity can buildup too much pressure, and too low viscosity can lead to break up of meltjet (described later) and no continuous fiber.

We turn now to the apparatus downstream of the syringe in addition todroplet forming passageway 20. A guiding chamber 25, e.g., of 5 to 12inches in diameter, is in fluid communication with the orifice outlet(s)of passageway 20. Polymer or nanocomposite fiber is formed andsignificantly elongated in chamber 25. Surrounding the guiding chamber25 is a glass heating duct 26 which is heated by hot air passingtherethrough which supplies heat to air in the interior of the chamber,also known as the whipping region, by conduction. Alternatively, thechamber 25 may be subjected to infrared heating. The temperature in theguiding chamber 25 is denoted T₃. A reason for heating in the guidingchamber 25 is to control the solidification of fiber being formed and topotentate flashing of any solvent. Too rapid cooling gives rise topremature solidification, whereas to slow cooling induces relaxation ofmolecular orientation; both lead to poor fiber properties. Moreparticularly, too rapid decrease in temperature T₃ leads to quenchingcrystallinity of crystallizing polymer and molecular orientation ofamorphous polymer of the fiber whereas too high a temperature breaks upthe fluid jet and/or induces relaxation which leads to poor fiberproperties. Conventional fiber melt spinning processes utilizeconvection by air blowing to control temperature in their spinningregions; in the instant case, air blowing that destroys the whippingmotion as described later, would antagonize proper fiber formation. Atthe rear end of chamber 25 is a collector 28 for collecting elongatedfiber which is formed. The fiber undergoing whipping motion is denoted30. The collector 28 is grounded as depicted at 32, so the voltage ofthe collector drops from tens of kV at the tip of the needle/spinneret20 to a few volts at the collector. A resistor R is included downstreamof the collector to enable measurement of the voltage of the collectorvia a meter 34. The temperature of collector 28 denoted T₄ providesannealing for the fiber on collection to provide more stable (no changesin properties with time) fiber with higher crystallinity forcrystallizing polymers and better molecular orientation for amorphouspolymers (and thus better properties). Too high a temperature T₄ willinduce relaxation. The temperature T₄ is provided by circulating waterthrough the interior of the collector from a temperature-controlled bath44 via feed and return lines 46 and 48. Ideally a controller is presentto control the temperature T₃ in response to results in the guidingchamber. The apparatus can be adapted from conventional melt fiberpreparation apparatus.

We turn now to operation of the apparatus of FIG. 1 for meltelectrospinning. Pellets of polymer or nanocomposite are introduced intosyringe 11 whereupon plunger 13 is inserted and micro-pump 18 ispositioned. The heating chamber 10 is heated. PLA of number averagemolecular weight of 186,000 and polydispersity 1.76 (determined by gelpermeation chromatography using polystyrene standards) obtained fromCargill-Dow was used in experiments herein. When the polymer is PLA, auseful temperature obtained in chamber 10/syringe 11 is 200° C. Thepolymer/nanocomposite is maintained in the syringe at preselectedmelting temperature for a period sufficient to obtain melting of all thepolymer/nanocomposite in the syringe, e.g., 30 minutes. Thereuponmicro-pump 18 is used to push polymer through syringe 11 to continuouslyimplement formation of a droplet(s) of polymer or nanocomposite, andhigh voltage source 22 effects a voltage, e.g., of 10 to 20 kV, at thetip(s) or orifices of 20 positioned 2 to 12 inches, e.g., 6 inches, fromthe collector 28 to effect an electrostatic field strength of 1 to 10kV/cm distance between tip and collector in the droplet to drive fiberforming. The field strength applied is sufficient to supply a charge toformed droplets which overcomes surface tension of the droplet(s), toproduce an electrified jet of molten polymer or nanocomposite to provideunstable flow, starting with axisymmetric modulation and progressing toa plurality of electrically induced bending instabilities/whippingmotions (repulsion of adjacent segments generates a vigorous whippingmotion) and stretching of the fibers which are being formed andproduction of solidified elongated fibers. For non-polar melts, e.g.,melts of polymers of polyolefins such as polyethylene (LDPE, LLDPE andHDPE), the end of the aforestated electrostatic field strength range forelectrospinning (5 to 10 kV/cm out of 1 to 10 kV/cm) is required. Thetemperature T₂ is provided by shielded electric resistance heater 41, toeffect low enough viscosity so there is not inappropriate pressurebuildup but not so low as to cause break-up of the melt jet (e.g.,200-230° C. for PLA). A temperature T₃ is provided which does not quenchthe fiber and does not break up the fluid jet or induce relaxation(e.g., 40 to 120° C. for PLA). The elongated solidified fiber isdeposited and collected on collector 28 which is maintained at atemperature T₄ by circulating temperature controlled water as indicatedat 44, 46 and 48 for annealing the fibers to provide more stable (nochange in properties with time) fibers with higher crystallinity forcrystallizing fibers and better molecular orientation for amorphousfibers (T₄ between room temperature and 80° C. was used for PLA).Typical annealing temperatures range from 60 to 120° C. and typicalannealing times range from 60 to 300 minutes. A volumetric flow rate of0.005 to 0.025 ml/min, typically 0.005 ml/min or 0.01 ml/min., was usedin experiments. In the experiments, the collector 28 was groundedaluminum foil on a metal sheet. During the processing, the temperatureT₃ is provided by circulating hot air in duct 26 to provide conductiveheating without the interference with the whipping motion that would beprovided by convective heating, and the heating chamber 10 used in theexperiments was constructed of thermally and electrically insulatingmaterial based on CaSiO₃ and heating coils in chamber 10 were surroundedby a Faraday cage, to prevent leaking of current into chamber 10 andinduction of voltage in the heating coils.

We turn now to the operation of the apparatus of FIG. 1 for elevatedtemperature solution electrospinning. Solvent and polymer ornanocomposite are homogenized in a high temperature oven (not shown) toform elevated temperature homogeneous solution. The plunger 13 isremoved from the syringe 11, the elevated temperature solution isintroduced into the syringe 11 and the plunger 13 is then inserted sothat any leakage is prevented. The syringe 11 with elevated temperaturepolymer or nanocomposite solution therein is placed in the heatingchamber 10. The temperature in the heating chamber 10 is controlled via12 and 15 to maintain the elevated temperature of the polymer ornanocomposite solution. The mini-pump 18 is activated to feed theelevated temperature polymer or nanocomposite solution throughneedle/spinneret 20. The temperature T₂ is provided by electricallyshielded heater 41 in response to controller 40 so as to maintain thepolymer solution at elevated temperature and viscosity such thatdroplets are formed in needle/spinneret 20. The high voltage source 22effects a voltage, e.g., 10 to 30 kV, at the tip(s) or orifice(s) of 20positioned 2 to 12 inches from collector 28. Voltage is not induced bythe high voltage applied at the tip(s)/orifice(s) in the heater coil ofheater 10 because of shielding in 10. An electrostatic field strength,e.g., of 1 to 10 kV/cm, where cm refers to the distance between dropletforming orifice of passageway 20 and the collector 28 is provided todrive the flow of polymer or nanocomposite solution to produce anelectrified fluid jet of polymer or nanocomposite solution and whippingaction. Just as in the case of melt electrospinning, whipping action andelongation of produced fiber occurs because of local variations ofsurface charges and electric field. The temperature control provided bycirculating hot air in jacket 26 of guiding chamber 25 provides atemperature T₃ that is not so low that quenching of the fiber isprovided and not so high that the fluid jet is broken or relaxation isinduced. In addition, a temperature 21 is provided to potentiateflashing off of solvent. In the experimental setup used in experimentsinvolving the invention, the guiding chamber is not a closed system andthe solvent evaporates and is vented through a hood. In a commercialsetup, an outlet is provided in the collection chamber for exit ofevaporated solvent and that outlet leads to a collection chamber outsidethe guiding chamber, so the recovered solvent can be recycled ordisposed of. After flashing off of solvent, the fiber is elongated andcollected as in the case of melt electrospinning operation describedabove.

We turn now to apparatus of the invention herein involving continuousoperation for melt electrospinning.

With continuing reference to FIG. 2 of the drawings, a hopper 50 isprovided for holding and feeding chunks of polymer or nanocomposite intoa melt extruder 52 which conveys and melts polymer or nanocomposite fedby hopper 50 and provides molten polymer or nanocomposite at its outlet.Heat is supplied in the extruder to melt the polymer or nanocomposite.Heat is provided in the extruder for melting, e.g., by indirect heatexchange, e.g. with steam or superheated steam circulating in a jacketfor the extruder. The melted polymer or nanocomposite from the extruderis pumped by force caused by the worm of the extruder via a pipe 54 tothe inlet of a melt pump 56 which is available as an item of commerce.The melt pump 56 contains a resistance heater (not shown) to maintainthe polymer or nanocomposite in molten form and force molten polymer ornanocomposite through a pipe 58 to a die header 60 containing multiplenozzles 65. A high voltage source 62 supplies high voltage, e.g., 10 kVto 30 kV where the distance from the nozzle outlets to a collector is 2to 10 inches, via a conductive element 63 to the nozzles 65 to providean electrostatic field strength, e.g., 1 to 10 kV/cm, where cm refers tothe distance from nozzle outlet to fiber collector. The electricalinsulation 64 on die header 60 shield the die frame and nozzles from theresistance heater of the melt pump 56 so voltage is not induced in thecoil of the melt pump 56. The nozzles 65 contain orifices whichcommunicate with a guiding chamber 66 which is heated by infrared (IR)apparatus (a IR chamber is being built composed of a ceramic infraredradiant heating panel on one side and a glass or metal reflector on theopposite side and the amount of IR radiation from the ceramic panel iscontrolled, e.g., by feedback of a thermocouple on the reflector, tocontrol the temperature in the chamber; alternatives for ceramic as theIR emissive heating medium are quartz and metal. At the outlet side ofthe guiding chamber 66 is a continuous collector 68 which can be amoving belt which can be in association with a heater moving at a speedconsistent with providing annealing.

To change the system of FIG. 2 to one for continuous elevatedtemperature solution electrospinning, mixer at elevated temperature isused in place of the melt extruder and solvent trapping apparatus isprovided outside of and in communication with the guiding chamber tocollect solvent.

Turning now to operation of the continuous system for meltelectrospinning, chunks of polymer or nanocomposite are fed from hopper50 to melt extruder 52 which provides at its outlet a melt of polymer ornanocomposite. The melt is delivered to melt pump 56 via pipe 54 and istransmitted through pipe 54 by pumping action of extruder 54 and suctionof melt pump 56. The melt pump 56 maintains the melt in melted conditionand at suitable viscosity for droplet forming. The melt pump 56 deliverspolymer or nanocomposite melt via pipe 58 to die header 60 and nozzles65. The high voltage source 62 supplies high voltage, e.g., 10 kV to 30kV where the distance from nozzle orifice to collector is 2 to 10inches, via conductive element 63 to the tips of nozzles 65. Theelectrostatic field produced thereby is shielded from the resistanceheater of melt pump 56 by electrical insulation 64. Droplets of moltenpolymer or nanocomposite are formed at the nozzle tips and the fieldstrength applied is sufficient to supply a charge to formed droplets, toprovoke electrical jets of molten polymer or nanocomposite and whippingaction to cause fiber formation and elongation. The IR heating inchamber 66 imparts a temperature above the quenching temperature of thepolymer or nanocomposite but below a temperature causing induction offluid jet disintegration or molecular relaxation in the fiber. Thecollector 68 is run at a speed such as to allow for collection of thefibers as a non-woven fabric and, if desired, annealing thereof.

Turning now to operation of the continuous system of FIG. 2 as modifiedfor solution electrospinning, polymer or nanoclay solution is formed inthe mixer at elevated temperature which is used in the place of the meltextruder. Otherwise, the operation is the same as the continuousoperation of melt electrospinning as described above, except thatsolvent evaporating in the guiding chamber 66 is collected and recycledor disposed of.

Fibers of relatively uniform size are obtainable herein. The fiberdiameter can be controlled by variation of needle/spinneret diameter,electric field strength (voltage/distance), infusion rate, distance fromnozzle to collecting surface, nozzle temperature and guiding chambertemperature. Experiments described below obtained fibers of diameter ofmicron size down to 150 nm. More recently, fibers of a diameter of about100 nm were obtained, that is nanofibers (fibers of diameter of 100 nmor less). For crystallizing polymers peaks associated with coldcrystallization and β crystal structure become more distinct as T₃decreases, and thus the crystallinity can be controlled by changingspinning temperature T₃. Experiments with PLA and PLA nanocompositesindicate that electrospinning induces β PLA crystal structure withfibrillar morphology.

The non-woven fabric formed in general has a specific surface arearanging from 10 m²/g to 1,000 m²/g and is useful, for example, forfiltration, protective clothing, biomedical applications, reinforcedcomposites, catalysts, and membranes. In experiments herein, 2″×2″ and5″×5″ non-woven mats of 100-500nm fibers were produced for evaluation.

The invention is illustrated in the following working examples. In theseexamples, the apparatus of FIG. 1 are used except that nozzletemperature T₂ was varied using circulating air. The experimentsinvolved melt electrospinning and the polymer employed was polylacticacid of number average molecular weight of 186,000 and polydispersity of1.76. The guiding chamber used was 10 inches in diameter. Annealingtemperature was 60° C. and annealing was carried out for 120 minutes.Flow rate, distance from orifice to collecting plate, applied voltage,T₂, T₃ and nozzle diameter were varied. The temperature to melt thepolymer in the heating chamber 10 was 200° C.

WORKING EXAMPLE I Effect of Flow Rate, Distance and Applied Voltage onFiber Diameter

The nozzle diameter was 0.84mm. The temperatures used were T₂=220° C.,T₃=100° C. and T₄=60° C. Flow rates, distance between nozzle orifice andcollector, voltage applied to the nozzle, are varied and results interms of fiber diameter in μm are given in Table 1 below.

TABLE 1 Voltage Flow Rate Distance 10 kV 15 kV 20 kV  0.01/ml/min 3″3.23 ± 0.67 5.34 ± 0.67 14.29 ± 2.83  6″ 7.65 ± 1.45 5.53 ± 0.91 8.21 ±1.77 0.005 ml/min 3″ 5.74 ± 1.45 4.85 ± 1.00 8.83 ± 1.66 6″ 6.67 ± 1.104.70 ± 0.94 4.46 ± 2.19

Except for one case with 10 kV and 3 inches, decreasing flow ratedecreases the fiber diameter, possibly due to the increase in residencetime (and thus, lower exposure to whipping motion).

At higher voltage setting (20 kV), the straight stable jet tends toextend longer and thus the whipping region is shortened, which leads tothicker fibers. Increasing the distance (from nozzle tip to collector)and thus increasing the whipping region, gives rise to thinner fibers.At lower voltage (10 kV), the electric field is weak and thus increasingthe distance decreases the whipping motion, leading to thicker fibers.At intermediate voltage (15 kV), these opposite influences on fiberdiameter seem to even out so increase in distance from nozzle tip tocollector results in almost no change in fiber diameter.

WORKING EXAMPLE II Effect of Nozzle Temperature (T₂) on Fiber Diameter

The nozzle diameter was 0.84 mm. The temperatures used were T₃=100° C.and T₄=60°; T₂ was varied. Flow rate was 0.01 ml/min. Voltage was 15 kV.Distance between the nozzle and collector was 3 inches. The results aregiven in Table 2 below:

TABLE 2 T₂ Fiber Diameter Standard (° C.) (μm) dev. (μm) 215 5.58 0.54225 6.17 2.19 190 6.85 0.46 160 9.49 1.13 175 5.76 1.12 205 5.36 1.70The results show that if T₂ is too high or too low, the fiber diametertends to get thicker. Too low temperature freezes up the filament andthus less whipping motion can be induced. Too high temperature decreasesthe viscosity of the jet, and eventually continuous production of fiberwould not be possible. High temperature (225° C.) also leads to poorsize distribution. From the data it appears that T₂ of above 215 to 220°C. leads to small fiber diameter with uniform size distribution.

WORKING EXAMPLE III Effect of (T3) on Fiber Diameter

The nozzle diameter was 0.84 mm. The temperatures used were T₂220° C.and T₄=60°; T₃ was varied. Flow rate was 0.01 ml/min. Voltage was 15 kV,and distance between the nozzle and collector was 3 inches. The resultsare given in Table 3 below:

TABLE 3 T₃ Fiber Diameter Standard (° C.) (μm) dev. (μm) 25 15.0 2.54100 5.34 0.67

The results show that increasing T₃ decreases fiber diameter, and withT₃=100° C., uniform size distribution is obtained.

WORKING EXAMPLE IV Effect of Nozzle Diameter on Fiber Diameter

The temperatures used were T₂=220° C., T₃=100° C., and T₄=60°. Flow ratewas 0.01 ml/min. Voltage was 15 kV. Distance between the nozzle andcollector was 3 inches. Nozzle diameter was varied. Results are setforth in Table 4 below.

TABLE 4 Nozzle Diameter Fiber Diameter T₃ (mm) (μm) (° C.) 0.84 15.0 250.30 6.35 25 0.15 2.95 25 0.12 1.51 25 0.84 5.34 100 0.30 2.26 100 0.151.05 100 0.12 0.54

The results indicate that the nozzle diameter significantly influencesthe average diameter of electrospun fibers. At a given spinningtemperature, the diameter gradually decreases with decreasing the nozzlediameter. However, the pressure drop required to feed the flowdrastically increases (the pressure drop is roughly proportianl to1/diameter²) as the nozzle diameter decreases.

WORKING EXAMPLE V Effect of Configuration of the Spinning Setup

The temperatures used were T₂=220° C., T₃=100° C., and T₄=60°. Flow ratewas 0.01 ml/min. Voltage was 15 kV. Distance between the nozzle andcollector was 3 inches. The electrospinning setup was varied. Resultsare set forth in Table 5 below.

TABLE 5 Spinning Fiber Diameter Standard Configuration (μm) dev. (μm)Vertical (downward) 10.0 1.76 Horizontal 5.34 0.67 Vertical (upward)0.85 0.57

The results indicate that the degree and extent of whipping duringelectrospinning decreases with increased effect of gravity. Hence, thefiber diameter becomes smaller as the whipping motion is more affectedby gravity. A smaller fiber dimension increases the ratio of surfacearea to volume (or mass) of electrospun mats (fabrics). Thus, smallerfiber dimension provides larger ratio of surface area to volume or massfor those applications where this is important, e.g. catalyticreactions, cell growth, etc. Moreover, smaller fiber dimension providesenhanced effects for filtration applications. For example, smallerfibers constituting filter media will collect smaller dust particleswithout increasing pressure drop, because of slip flow at small fiberinterface. Hence, filtration efficiency increases with smaller dimensionfibers.

Variations

The foregoing description of the invention has been presented describingcertain operable and preferred embodiments. It is not intended that theinvention should be so limited since variations and modificationsthereof will be obvious to those skilled in the art, all of which arewithin the spirit and scope of the invention.

What is claimed is:
 1. Apparatus for elevated temperature production ofnon-woven fabric from thermoplastic polymer or thermoplastic polymernanoclay nanocomposite, neat or in solution and requiring elevatedtemperature for dissolving in an acceptable solvent, said apparatuscomprising a resistance heater for melting the polymer or nanocompositeor maintaining the polymer or nanocomposite in solution in acceptablesolvent; a pump upstream of or containing the resistance heater forcausing dispensing of said melted polymer or nanocomposite or elevatedtemperature solution; a droplet forming passageway for receiving saidpolymer or nanocomposite melt or elevated temperature solution andhaving one or more outlet orifices for providing one or more droplets ofmelted polymer or nanocomposite or elevated temperature solution at theone or more outlet orifices; a guiding chamber having an inlet side influid communication with the outlet orifice(s); a collection surface ata rear side of the guiding chamber for receiving elongated fibers ofpolymer or nanocomposite and collecting them as a non-woven fabric; anda high voltage source in electrical communication with the dropletforming passageway to provide an electric charge in the droplet(s)emitting therefrom to overcome the surface tension of a droplet toproduce a jet of melted polymer or nanocomposite or elevated temperaturesolution in the guiding chamber giving rise to unstable flow through theguiding chamber to the collection surface manifested by a series ofelectrically induced bending instabilities and flashing off of anysolvent during passage of polymer or nanocomposite to the collectionsurface and production of elongated fibers of polymer or nanocompositeand deposit of these on the collection surface so as to form thenon-woven fabric.
 2. The apparatus of claim 1 where the electricalcommunication of the high voltage source is shielded from the resistanceheater to prevent induced voltage in the resistance heater and where atemperature modulator is provided for the guiding chamber to adjustcooling of the fiber being formed to provide against prematuresolidification and to provide against induction of relaxation ofmolecular orientation, and to potentiate flashing off of any solvent,without affecting the bending instabilities causing fiber elongation. 3.The apparatus of claim 1 which is for batch operation, said apparatuscomprising the following elements: (a) a syringe having an inlet forintroduction into the syringe of solid meltable thermoplastic polymer orsolid meltable thermoplastic polymer nanoclay nanocomposite or solutionof thermoplastic polymer or thermoplastic polymer nanoclay nanocompositerequiring elevated temperature for dissolving, and an outlet fordispensing of melted thermoplastic polymer or nanocomposite or elevatedtemperature solution, (b) a heating chamber in heat exchangecommunication with the syringe to supply heat to the syringe to meltpolymer or nanocomposite or maintain polymer or nanocomposite insolution within the syringe, (c) droplet forming passageway having aninlet in fluid communication with the outlet of the syringe and one ormore outlet orifices for providing one or more droplets of polymer ornanocomposite melt or elevated temperature solution at the one or moreoutlet orifices; (d) a pump upstream of the inlet of the syringe forcausing the syringe to dispense melted polymer or nanocomposite orelevated temperature solution to the droplet forming passageway, (e) aguiding chamber having inlet side in fluid communication with theorifice outlet(s); (f) a collection surface at a rear end of the guidingchamber; and a high voltage source in electrical communication with thedroplet forming passageway to provide an electric charge in thedroplet(s) emitting therefrom to overcome the surface tension of adroplet to produce a jet of melted polymer or nanocomposite or elevatedtemperature solution in the guiding chamber giving rise to unstable flowthrough the guiding chamber to the collection surface manifested by aseries of electrically induced bending instabilities and flashing off ofany solvent, during passage to the collection surface, and production ofelongated fibers of the polymer or nanocomposite which are deposited onthe collection surface where they are collected as a non-woven fabric.4. The apparatus of claim 1 which comprises at least one of thefollowing elements (h), (i) and (j): (h) a temperature modulator for theguiding chamber to adjust cooling of the fiber being formed to provideagainst premature solidification and to provide against induction ofrelaxation of molecular orientation and to potentiate flashing off ofany solvent, without affecting the bending instabilities causing fiberelongation, (i) a controller for controlling temperature in the heatingchamber, a heating coil in the heating chamber, and shielding for theheating coil inside the heating chamber to prevent induced voltage inthe heating coil from the electric charge supplied by the high voltagesource so that induced voltage will not affect or damage the controller,(j) the heating chamber being constructed of material comprising asubstance that provides both thermal and electrical insulation.
 5. Theapparatus of claim 4 which includes a modulator for the temperature ofthe collection surface to provide annealing of fibers deposited on thecollection surface to provide fibers on the collection surface withproperties that do not change with time and have increased molecularorientation.
 6. The apparatus of claim 1 which is for continuous meltelectrospinning operation, and for production of non-woven fabric fromthermoplastic polymer or thermoplastic polymer nanoclay nanocomposite,comprising a hopper for containing and feeding chunks of thermoplasticpolymer or thermoplastic polymer nanoclay nanocomposite; an extruder forreceiving the polymer or nanocomposite from the hopper and conveying,melting and pumping the polymer or nanocomposite to produce a flow ofpolymer or nanocomposite melt therefrom; a melt pump for receiving themelted polymer or nanocomposite from the extruder and for maintainingthe melted condition of the polymer or nanocomposite melt by means ofelectric resistance heating and providing a melt output; a header forreceiving the melt output and distributing it to multiple nozzles forforming droplets of polymer or nanocomposite melt; a guiding chamber forreceiving the output of the nozzles, a collection surface at a rear endof the guiding chamber; and a high voltage source in electricalcommunication with the nozzles to provide an electric charge in thedroplets emitting therefrom to overcome the surface tension of a dropletto produce a jet of polymer or nanocomposite melt giving rise tounstable flow through the guiding chamber to the collection surfacemanifested by a series of electrically induced bending instabilitiesduring passage to the collection surface and production of elongatedfibers of the polymer or nanocomposite which are deposited on thecollection surface where they are collected as a non-woven fabric, ashield for the header and nozzles to prevent induced voltage in the meltpump from the electric charge supplied by the high voltage source; andan infrared heater for the guiding chamber to adjust cooling of thefiber formed therein to provide against premature solidification and toprovide against induction of relaxation of molecular orientation,without affecting the bending instabilities causing fiber elongation. 7.A method for melt electrospinning production of non-woven fabric frommeltable thermoplastic polymer or meltable thermoplastic polymernanoclay nanocomposite, said method comprising the steps of (a) meltingthermoplastic polymer or nanocomposite in a melting zone, (b) moving thethermoplastic polymer or nanocomposite through the melting zone by aforce supplier upstream of or in the melting zone, (c) forming dropletsfrom the melted polymer or nanocomposite, (d) providing an electriccharge on the droplets to overcome the surface tension of a droplet toproduce a jet of melted polymer or nanocomposite and provide unstableflow involving a plurality of electrically induced bendinginstabilities/whipping motions and elongation of and production ofpolymer or nanocomposite fibers, (e) collecting of the elongated fibersto form a non-woven fabric.
 8. The method of claim 7 additionallycomprising at least one of the following steps (f) and (g): (f)providing a temperature for the polymer or nanocomposite being subjectedto electrically induced bending instabilities/whipping motions and fiberelongation so as to provide against premature solidification and toprovide against induction of relaxation of molecular orientation withoutaffecting the electrically induced bending instabilities, (g) shieldingto prevent induction of voltage in the melting zone.
 9. The method ofclaim 8 comprising the additional step of annealing the collected fibersto impart stability and molecular orientation.
 10. A method for hightemperature solution electrospinning of non-woven fabric fromthermoplastic polymer or thermoplastic polymer nanoclay nanocompositethat is not dissolvable at room temperature in an acceptable solvent,said method comprising the steps of: (a) homogenizing the polymer ornanocomposite in solvent in an elevated temperature zone to form asolution of the polymer or nanocomposite in the solvent; (b) maintainingthe solution at a temperature sufficient for maintaining dissolution ina second elevated temperature zone; (c) moving the solution through thesecond elevated temperature zone by a force supplier upstream of or atthe second elevated temperature zone; (d) forming droplets of thesolution moved through the second elevated temperature zone; (e)providing an electric charge on the droplets to overcome the surfacetension of a droplet to produce a jet of polymer on-nanocompositesolution and provide unstable flow involving a plurality of electricallyinduced bending instabilities/whipping motions and flashing off ofsolvent and elongation of and production of polymer or nanocompositefibers; (f) collecting the fibers to form a non-woven fabric.
 11. Themethod of claim 10 additionally comprising at least one of the followingsteps (g) and (h): (g) providing a temperature for the polymer ornanocomposite and solution thereof being subjected to electricallyinduced bending instabilities/whipping motions and fiber elongation toprovide against premature solidification and to provide againstinduction of relaxation of molecular orientation and potentiate flashingoff of solvent, without affecting the electrically induced bendinginstabilities, (h) shielding to prevent induction of voltage in thesecond elevated temperature zone.
 12. The method of claim 11, comprisingthe additional step of annealing the collected fibers to impartstability and molecular orientation.
 13. A fiber formed from a polymer,wherein a temperature in a droplet forming passageway that the polymerpasses through to form droplets is above 215° C.
 14. The fiber of claim13, wherein the polymer is a meltable thermoplastic polymer.
 15. Thefiber of claim 14, wherein the meltable thermoplastic polymer isselected from the group consisting of rubber, polycarbonate,polystyrene, poly(methyl methacrylate), poly(lactic acid), polyethyleneterephthalate, polybutylene terephthalate, nylon 6, polypropylene,polyethylene, and nylon 6,6.